I made this with the memory of the gyudon (beef rice bowl) I had at a certain restaurant a long time ago.
You can make gyudon any time as long as you have some onions and beef, so it's a easy to make life saver. I've been making this way for a long time.
You can use a cheap cut of beef, but choose a type that has plenty of fat and is thinly sliced for the best flavor. Please adjust the amount of sugar and soy sauce to your liking.
Enjoy Japanese food around the world.